The need exists for small, safe on-site reactors to produce synthesis gas, that is a gas containing carbon monoxide and hydrogen, for use in subsequent chemical reactions or as a fuel. Synthesis gas is produced by known partial oxidation and steam methane reforming reactions or a combination of the two known as autothermal reforming.
In a partial oxidation reaction, a hydrocarbon containing stream, for instance, natural gas, oxygen and, optionally, steam are introduced into a partial oxidation reactor with the use of a specially designed burner. The oxygen is consumed at the reactor entrance. The residence time in the reactor is about 3 seconds. The overall reaction that takes place is:CH4+0.5O2═CO+2H2 The following side reactions also occur.CH4+2O2═CO2+2H2OH2+0.5O2═H2OCO+0.5O2═CO2 The side reactions are undesirable and reduce the product H2:CO ratio from a stoichiometric ratio of 2 to a ratio ranging from about 1.7 to about 1.8. The initial reaction is exothermic and produces heat and consequential temperature increases to above about 1300° C. The high temperatures allow the following reforming reactions to occur at the main part of the reactor:CH4+H2O═CO+3H2 CH4+CO2=2CO+2H2 CO2+H2═CO+H2OThe crude synthesis gas is treated in a separation system to recover a hydrogen or carbon monoxide product. Common separation systems that are employed for such purpose include: membrane separation units; MEA adsorption units, PSA, and cryogenic separation units.
In steam methane reforming, the hydrocarbon containing stream, steam and, optionally, a recycle stream, primarily containing carbon dioxide, is fed into a reactor. Commonly the reactor is formed by a bundle of tubes containing a catalyst. The tube bundle is located in a furnace and natural gas is also used as a fuel to the furnace. The hydrocarbon containing stream to be reacted is pretreated to remove sulfur which is a poison with respect to the reforming catalyst. Typically, the sulfur level in the natural gas is reduced to a part per million level before the natural gas is fed into the reactor. The following reactions take place inside the catalyst packed tubes:CH4+H2O═CO+3H2 CH4+CO2=2CO+2H2 CO2+H2═CO+H2OThe crude synthesis gas product from the reactor, which contains hydrogen, carbon monoxide, and water, is cooled down to avoid the re-reforming of methane from the carbon monoxide and the hydrogen. The crude synthesis gas may be treated in a variety of separation units to remove impurities or to recover a desired product such as hydrogen or carbon monoxide in a manner that is similar to that provided for in a partial oxidation reactor.
In autothermal reforming, in a first reaction zone formed by a burner, natural gas, oxygen and, optionally, steam and a recycle stream containing CO2 are reacted. The reactions in this first reaction zone are as follows:CH4+2O2═CO2+2H2O H2 
The resultant intermediate product from the first reaction zone containing methane, water, and carbon dioxide, is fed to a catalyst bed below the burner where the final equilibration takes place in the following reactions:CH4+H2O═CO+3H2 CO2+H2═CO+H2OCH4+CO2═CO+H2 The catalyst bed may be a vessel filled with catalyst as disclosed in U.S. Pat. No. 5,554,351 or a fluid bed catalyst system such as disclosed in U.S. Pat. No. 4,888,131. In the fluid bed system disclosed in the aforesaid patent, methane and steam are fed to the bottom of the fluid bed and oxygen is fed close to the bottom but inside the fluid bed. The crude synthesis gas can be treated in separation systems such as have been discussed above with respect to partial oxidation units.
The prior art reactors, such as those discussed above, are designed for large volume production of synthesis gases. They are not amenable to be modified for production on a lower scale in that they require furnaces, fluidized beds, burner, complex controls and etc. U.S. Pat. No. 6,471,937 discloses a reactor that by its very nature is capable of being utilized in lower volume applications. In this patent, two reactants are mixed by expanding one of the reactants into a mixing chamber through an orifice and entraining the other reactant to form the mixture. The mixture is formed in such a sufficiently short time that no reaction takes place between the two reactants. The reactants, once mixed, are then reacted with one another in a reaction zone. The problem with this type of reactor is that although it is desirable to use a high surface area high activity catalyst in reality if the reaction is highly exothermic the lifetime of such catalyst will be severely limited.
As will be discussed, the present invention provides a reactor and method that employs an autothermal reactor in which reactants are mixed without reaction, in a manner that can take the form outlined in U.S. Pat. No. 6,471,937, and that is designed to produce a sufficient amount of synthesis gas product to allow such reactor and method to be practically utilized for small, on-site production of synthesis gases.